Why Pain Gets Worse: The Mechanism of Heat Hyperalgesia

摘要

Pain differs from other senses in many ways; one of the most striking is that the intensity of pain increases with time in the presence of a painful stimulus, a process that is referred to as sensitization or hyperalgesia. With all the other senses, the perceived intensity decreases with continuous exposure to a constant stimulus. For instance, when you emerge from a cinema onto a sunny street you are at first dazzled, but soon adapt to the new and higher ambient light level. If vision behaved like pain the opposite would happen—the dazzling light of the street would become ever brighter. The reason for the difference is clear enough. Light adaptation allows us to operate over a wide range of ambient light intensities, a necessary property for any species that may at one moment be in bright sunlight and the next in a dark cave (possibly containing a bear). Pain, on the other hand, cannot be ignored as it often signals tissue damage, and it is essential for the integrity of the organism that a painful stimulus should become ever more pressing until the subject takes some action to remove its cause.